1. Field of the Invention
This invention relates to solid state power contacts for switching a-c power in response to logic signals and particularly to such power contacts having low leakage characteristics, yet also being capable of switching sizable currents. The invention further relates to power contacts in which the logic circuit controlling the switch is electrically isolated from the a-c circuit being switched and in which the contact is protected from surges in the a-c circuit.
2. Prior Art
It is common today to use low level d-c logic signals to control a-c power circuits. For instance, in many applications the output of a digital computer is used to control components energized by an a-c power source. These components can include a wide range of devices from highly inductive loads such as motors which draw a substantial amount of current to indicators such as neon bulbs which only draw 1 to 2 milliamperes of current. It is highly desirable to have to provide only one type of power contact to accommodate all of these types of loads.
However, these various loads impose their own sometimes opposing limitations on the power contact. The highly inductive loads generate switching transients which must be suppressed to protect the contacts. The load devices operated by very low currents require that the power contacts have very low leakage current when in the off state in order to deenergize these devices. Unfortunately, the devices normally used to suppress the transients caused by the inductive loads add to the leakage current. The whole problem is further compounded in instrumentation circuits which must meet IEEE surge protection specifications.
Relays are one type of device used as power contacts in computer interfaces. In addition to being comparatively slow, consuming substantial power and not being as dependable as solid state switches, relays require sizeable RC snubber circuits across the contacts to suppress the transients generated by inductive loads. In one application where the leakage current through the snubber was substantial enough to cause a stepper motor being controlled to skip positions, a rectifier bridge circuit was inserted between the a-c circuit and the relay so that the contacts switched d-c current to control the a-c current.
Solid state switches such as back to back thyristors and triacs are also used to control a-c circuits with d-c logic signals. These devices also require RC snubbers to suppress switching transients in the a-c circuit which also generate a-c leakage currents unacceptably high for the very low current loads such as neon indicator bulbs.